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Boston Harbor & Rivers
October 2009
MWRA monitors water quality at more than
50 locations in Boston Harbor and its three largest tributary rivers:
the Charles, Mystic, and Neponset. Monitoring
is conducted year-round on a rotating schedule, with
the most intense sampling in spring and summer.
In general, the tributary rivers have poorer
water quality than the harbor, reflecting the impacts of urban storm runoff
and combined sewer overflows (CSOs). These three rivers are dammed near
their entry to the harbor, which concentrates
nutrients and pollutants entering the rivers from upstream.
Among other water quality indicators,
MWRA measures E. coli, Enterococcus, fecal coliform, algae, and clarity. Moderate levels of algae and water clarity
are essential to a healthy harbor ecosystem. High fecal coliform counts
signal potential public health threats. Water quality indicators are likely
to be affected by environmental factors like temperature and rainfall
as well as discharges of contaminated stormwater or CSOs.
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THIS MONTH'S PRECIPITATION
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E. COLI & FECAL COLIFORM
E. coli and fecal coliform are bacteria found in human
and animal waste, measured in recreational waters to indicate
bacterial water quality and to assess public health risk. E. coli counts
greater than 126 colonies per 100 milliliters of water fail to meet
the Massachusetts Department of Public Health swimming standard
and indicate poor water quality. High levels of E. coli can occur following heavy rains that carry untreated waste into
rivers and the harbor from storm runoff and combined sewer overflows.
There are no longer State or EPA guidelines for fecal coliform in marine waters. However, like Enterococcus, it is a useful indicator of bacteria and/or viruses that originate in the human or animal digestive system. They indicate that pathogenic microorganisms may present a health risk for swimmers and those consuming shellfish. Sources of fecal contamination to surface waters include storm runoff, wild animal manure, and untreated combined sewer overflows (CSO). 
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E. Coli is no longer measured in Boston Harbor
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(Click on image to magnify)
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ENTEROCOCCUS
A type of bacteria present
in the intestines of warm-blooded animals, is an indicator of human sewage. During the swimming season,
a geometric mean count above 35 colonies per 100 mL (an average
of counts from samples collected over several days or weeks) results
in prolonged closure of a swimming area.
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(Click on images to magnify)
ALGAE
Chlorophyll is measured to determine the amount of algae in the water. In the harbor and rivers, photosynthesis is carried out by algae (or phytoplankton), microscopic plants suspended in the water column. High chlorophyll concentrations indicate an overabundance of nutrients in the water, which can result in elevated algae levels, or algal blooms. Algal blooms can deplete bottom-water dissolved oxygen, reduce water clarity, and impair recreational uses. Chlorophyll concentrations greater than 12 micrograms per liter in the Harbor and 25 micrograms per liter in the rivers indicate an overgrowth of algae.
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(Click on images to magnify)
WATER CLARITY
Water clarity in the harbor and the rivers
is primarily affected by concentrations of algae and suspended solids. Secchi disks are a simple way to approximate the transparency of
water. White or black-and-white disks are lowered into the water
and the maximum depth at which they are visible is recorded. Large
secchi disk depths indicate good water clarity. Secchi disk depths
less than 1.8 meters indicate poor water clarity.
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(Click on images to magnify) |
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